Dude, 1-Balpa(for advice), 2-EASA(to report them), 3-European court...too much money to let that go unchallenged.
Keep copies of every communication you had with that school,and receipts as well as your log books.Any "dirt" you might have on them(bad maintenance,sop's lacking,etc..)anything,use it against them when contacting EASA,remember it is a "Dog eats dog world".
Forget the Greek CAA and their justice system,it will only work if they are pressed.
Good luck!